Causes of ADHD - Genetics

Margaret V. Austin, Ph.D.

What Causes ADHD?

Researchers investigate the causes and cures of many diseases and disorders. Thus, ongoing ADHD research has attempted to answer the question, "What causes ADHD?" Because of this research, we now know a great deal more about ADHD than we did 10 years ago (See section on New and Exciting Brain Research). Despite this large body of research, the specific cause of this disorder remains uncertain.

The vast majority of researchers conclude that ADHD is primarily a neurological or brain-based disorder. It is either present at birth, or it develops early on in childhood. Although our everyday surroundings can affect the severity of symptoms, these environmental factors do not seem to be the primary cause of the disorder.

Is ADHD inherited and caused by genetics?

Research has established that genetics play a powerful role in many behavioral symptoms. These include behavioral symptoms observed in ADHD: impulsiveness, hyperactivity, and inattention. The estimated heritability of ADHD (i.e., the proportion of a trait that can be attributed to genetics) ranges from 75 to 91%. This does not mean a parent of an ADHD child must have ADHD. It does mean that the gene for ADHD exists in the family line of one of the biological parents. The specific gene has not yet been identified.

Twin studies support a genetic basis for ADHD. Concordance rates (the occurrence of similar traits) are higher among identical twins (58-82%) than fraternal twins (31%-38%). (Martin, Scourfield, & McGuffin, 2002). Interestingly, when one twin has ADHD, but the other does not, the 'normal' twin is still more likely to have problems with impulse control. Although genetics play a key role in ADHD, the specific genes have not yet been identified.

While twin studies provide the strongest evidence of a genetic influence, studies of brain chemistry lend further support. These studies investigate what happens in the brain when people take medications used to treat ADHD symptoms. Stimulant drugs increase brain activity. Initially, experts thought ADHD was a result of an overactive brain. Therefore, it seemed counter-intuitive to give stimulant drugs to someone who has problems with inattention, hyperactivity, and impulsivity. The unexpected, calming effect of these drugs baffled experts. To explain this paradox, experts theorized that stimulant medication must have some unique and opposite effect on children with ADHD.

It turns out that ADHD is not a result of an overactive brain as once assumed. In fact, quite the opposite is true. Research has determined that ADHD is actually a result of reduced brain functioning (particularly frontal lobe) and decreased levels dopamine. Dopamine is a naturally occurring chemical in the brain that creates sensations of arousal and pleasure. In other words, children with ADHD have cognitive 'sluggishness,' or a slower thinking process than their peers. In children with ADHD, stimulants raise the brain activity to a level more comparable to their peers. In people with ADHD, stimulants also increase their ability to screen out irrelevant stimuli, control impulsive behavior, and to focus on the designated task.

Can parents help prevent ADHD in their children?

The short answer is no. Parents cannot 'prevent' ADHD any more than they can prevent their hair color: It's genetic. However, there are many things caregivers and families can do to enable the success of their ADHD children.

That said, there are ways to help children to do their best. Since complications of pregnancy have been linked to ADHD, good prenatal care is a great way to start looking after your child's health. Eat healthy, avoid alcohol and drugs, and see your doctor regularly to make sure you are doing everything within your power to give your baby a healthy start in life.

Are there other causes of ADHD?

ADHD is recognized as a genetic condition that is passed from parents to children. As no one can choose their genetics, there is nothing parents can do to prevent ADHD. However, caregivers and teachers can do things than can lessen the severity of ADHD symptoms. Likewise, various situational factors can worsen ADHD, or create similar symptoms in people without the disorder. For instance, a calm, quiet environment promotes concentration while a noisy, chaotic location diminishes our ability to focus on any one thing.

Do prenatal and postnatal factors affect ADHD?

The root causes of ADHD are biological. Nonetheless, certain factors may increase the severity of symptoms. These are: prenatal and perinatal complications (e.g., low birth weight, fetal distress); prenatal alcohol or tobacco exposure; lead poisoning and pesticides; moderate to severe protein deficiency; head injuries that involve the frontal lobe; allergies; abuse or other violent trauma. These stressors must be examined more fully to determine their exact connection, if any, to ADHD.